To form an oil or gas well, a bottom hole assembly (BHA), including components such as a motor, steering assembly, one or more drill collars, and a drill bit, are coupled to a length of drill pipe to form a drill string. One or more sensors may be positioned within the drill string for making various downhole measurements. The sensors so positioned may include Geiger-Müller tubes for measuring radiation, including gamma radiation. Once assembled, the drill string is then inserted downhole, where drilling and data collection by the sensors commence.
In some conventional systems, one or more Geiger-Müller tubes are embedded within a chassis, and the chassis is inserted within a drill string tubular. For example, FIG. 1 depicts a conventional system 20, shown in cross-section, having a plurality of Geiger-Müller tubes 22 embedded within a chassis 24. Chassis 24 is inserted into a drill collar 28 and includes a flowbore 26 to permit the passage of drilling fluid therethrough. During operation of system 20, drill collar 28 protects tubes 22 from high pressure loads of drilling fluid passing through flowbore 26 and subsequently returning to the surface through an annulus between drill collar 28 and a surrounding wellbore, as well as mechanical loads from the drill string. A potential drawback to system 20 is that the thickness of drill collar 28 disposed between tubes 22 and radiation surrounding drill collar 28 may degrade the quality of the measurements taken by tubes 22.
Further, in some conventional systems, the Geiger-Müller tubes are spaced azimuthally about only a portion of the chassis, in contrast to system 20 which includes tubes 22 distributed azimuthally about the full circumference of chassis 24. For example, FIG. 2 depicts a conventional system 10, also shown in cross-section, having a plurality of Geiger-Müller tubes 12 embedded within and distributed azimuthally over a limited portion of a chassis 14. Chassis 14 is inserted into a drill collar 18, and includes a flowbore 16 to permit the passage of drilling fluid therethrough. In addition to the potential degradation of measurements taken by tubes 12 due to the thickness of drill collar 18, other potential drawbacks to system 10 include the reduced number of tubes 12, as compared to that of system 20. The reduced number of tubes 12 results in fewer measurements, which, in turn, may yield statistically inconsistent measurements. Additionally, the limited distribution of tubes 12 yields measurements of radiation levels proximate only a portion of drill collar 18, rather than entirely surrounding it. As such, circumferential variations in the radiation level around drill collar 18 may not be detected.